The present invention relates to polyimide foams prepared by the reaction between polyisocyanates and dianhydrides, and to the use of such foams in filling the voids in a cellular material such as honeycomb cell networks used as lightweight materials of construction.
Rigid cellular materials are used in a wide range of applications where low density must be maintained. The aerospace industry, for example, uses such materials extensively. For added strength and durability as well as control of properties, the cells are filled with a polymeric foam. Polyimide foams formed in such materials by the reaction of polyisocyanates and dianhydrides are desirable since the foaming is caused by the generation of carbon dioxide rather than water, thus avoiding the need for insuring that water is removed entirely prior to quenching the reaction. Unfortunately, the precursors for this type of reaction are generally in the slurry or semi-liquid state, which makes transportation and handling difficult, particularly when one seeks to perform the foaming reaction in place inside the cell structure. Externally prepared foams may be inserted in the cells either by first being pulverized to a granulated form or by being forcefully inserted as a continuous mass, in which case they are cut to size by the cell structure itself. These are awkward, unreliable procedures and are difficult to control.